Beat Sarkan the Mad for the second time in the campain (level 8). You can do it in the lower difficulty setting.
He uses a Black/Red deck that is not as good as some of the decks that you have if you own the other expansion packs. See below for an option if you don`t have the other packs.
Fast decks that can kill creatures are the ones that are effective against him. I beat him miserably using the Relics of Doom deck and the Ears of the Elves deck was also effective. Other decks you may try are the Eyes of Shadow (use Terror to kill his Red creatures and Mind Rot to keep his hand empty) and Heat of Battle (just burn everything he summons).
A less effective option is the Thoughts of Wind deck if you manage to have counters for the big creatures he summons.